Get in plank position with both feet on the floor, the right hand on a medicine ball or plyobox, and the left hand on the floor. Tilt the hips into posterior pelvic tilt and maintain a rigid plank for the duration of the exercise.

Slowly descend until the right hand is approximately aligned with the chest.

Press your body up to the starting position. 

DO NOT allow the hips or lower back to dip. DO NOT let go of the rigid plank-- body is a straight line the entire time.